Field of the Invention
The invention concerns a rotation rate sensor with a fiber-optic Sagnac interferometer as well as a method for controlling a rotation rate sensor with a fiber-optic Sagnac interferometer.
Description of the Prior Art
Fiber-optic Sagnac interferometers are used in rotation rate sensors in inertial navigation systems. Inertial navigation systems can be realized in various manners. Typically positioning is based on measuring forces or accelerations acting on an object and on applied rotation rates. Optical effects may be used instead of mechanical effects for determining rotation rotates for inertial navigation systems. Such an internal navigation system may be based on at least one fiber-optic Sagnac interferometer. This uses the Sagnac effect according to which an optical path difference occurs between two light beams running in opposite directions inside the light guide loop during a rotation about its normal. During observation and superposition of the two light beams exiting the light guide loop a change in intensity becomes visible, which can be described by an interferometer characteristic. This describes the intensity change in dependence of the phase difference between the two light waves. Differently stated, a rotational movement acting on the Sagnac interferometer results in a phase shift between the two light beams circulating opposite to each other such that, at the position where the beams are superposed, a change in intensity can be observed depending on the rotational movement.
The phase shift in a fiber-optic Sagnac interferometer is directly proportional to rotation speed, path length of light in the light guide loop or a light guide coil and the diameter of the circular light path. The phase shift is inversely proportional to the wave length of the light.
The above-described interferometer characteristic, which describes the dependency of light intensity and which is observable for determining the rotation from the phase difference, is cosine-shaped.
As a transfer function is insensitive to small input values in the maximum of the cosine curve and the sign of the phase shift corresponding to the rotation direction is not detectable, the working point of the Sagnac interferometer is often adjusted to the point of a maximum gradient of the cosin function. Sine- or square-wave modulation may be considered for this. Maximal sensitivity of the interferometer should be guaranteed at a small rotational movement.
A rotation rate sensor, which comprises a fiber-optic Sagnac interferometer, usually comprises a multi-functional integrated optical chip (MIOC) for carrying out a phase modulation. The MIOC is typically part of a control loop for adjusting the phase modulation described above. Physical effects in the MIOC, e.g. movable charge carriers, cause a frequency dependence of phase modulation. The MIOC therefore has a frequency response as different frequency responses occur for different frequencies. As this phenomenon can also be traced to the mobility of charge carriers in the MIOC, there is also a dependency of the MIOC frequency response on the surrounding temperature or the temperature of the MIOC itself.
This MIOC frequency response leads to a special form of the lock-in effect, which manifests itself such that, at a small rotational rate of about 0°/h an accumulation of output signals occurs that corresponds to a rotation rate of 0°/h. The insensitivity of the Sagnac interferometer with respect to very small rotation rates resulting therefrom constrains a highly precise and reliable measurement of, in particular, very small rotation rates under non-constant temperature conditions.